A shove converted to a spin
How Does a Bang Turn a Wheel?
Burning fuel shoves a piston down a tube, and a crankshaft turns that shove into rotation.
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Step 1 of 5
Burning fuel makes gas expand suddenly.
A spark lights a fine mist of fuel and air, and the burning gases try to occupy far more space than they did a moment earlier.
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Step 2 of 5
The expansion pushes a piston down a tube.
The only direction the gas can escape is by shoving the piston, a tight-fitting plug sealed by springy rings, down the cylinder.
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Step 3 of 5
A crank turns a push into a rotation.
The rod below the piston connects to a pin set off to one side of the shaft, so pushing the rod down forces the shaft to roll round.
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Step 4 of 5
Four strokes, over and over.
Draw in fuel and air, squeeze it tight, burn it for the one stroke that makes power, then push the waste gas out and start again.
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Step 5 of 5
Most of the fuel's energy escapes as heat.
Only a modest share of each bang becomes movement. The rest leaves through the exhaust pipe and the radiator without ever turning a wheel.
A petrol engine turns only about 25% of its fuel's energy into movement, so three quarters of every tankful leaves as heat.
The short version
A car engine burns fuel to shove a piston down a tube, and a crankshaft converts that straight shove into the spin the wheels need.
Try it yourself
Push a bicycle pedal down from the top and watch the crank. Your straight push becomes a turn, exactly as a crankshaft does.
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